Python -> Java -> D ?

[David Finlayson]

I am a Python scripter in search of a language with better performance. I do a lot of signal processing in my job and I think I've pushed Python as far as it will go. Recently, I have written a few programs in D and have been impressed with (a) how easy the transition from Python has been, (2) the large boost in performance for my code. 

Here's the question:

Should my second language be a new language like D? Or, would I be better off mastering something more mainstream such as C or C++?

The reason I ask is that D is similar enough to Python in terms of abstraction that I might be better off in the long run mastering something lower level such as C or C++, or higher level such as a good functional language like Lisp or Ocaml. 

I've noticed D is still quite immature in some ways. It has no printed books (a sign of market share) as compared to the dozen os so books written about C/C++ in my specific scientific discipline alone.  Also,  the standard library is in flux (Phobos vs Tango) whereas more mature languages have settled down.

In short, I like D, but maybe I would be better of learning something else. So, some of you alpha-geeks on this list: would you learn D next in my position or something else?